bunpen: use stdlib log isntead of raw fmt::printfln

pkgs/additional/bunpen/src/main.ha

@@ -1,12 +1,12 @@
 // vim: set shiftwidth=2 :
-use fmt;
+use log;
 use strings;
 use os;
 use os::exec;
 
 fn do_exec(args: []str) never = {
   let joined = strings::join(" ", args...);
-  fmt::printfln("exec: {}", joined)!;
+  log::printfln("exec: {}", joined);
   free(joined);
 
   let cmd = os::exec::cmd(args[0], args[1..]...)!;